The use of motor driven compressor units for elevating the pressure of a flow of gas is well distributed throughout the industrial community. Such compressors are available from a number of equipment manufacturers, and use a wide variety of mechanical principles for accomplishing the desired pressurization.
A significant fraction of these units are of the reciprocating piston type, wherein a compressor piston is reciprocated within a close fitting cylinder which is enclosed about at least one end. Such units accept low pressure gas into the cylinder as the piston is withdrawn, and push the gas out of the cylinder at a higher pressure as the piston is driven inward toward the closed end. The flow of gas into and out of the closed portion of the cylinder-piston combination is directed by a plurality of gas valves which may be manipulated mechanically by linkages or the like. As is well known to those skilled in the art of gas compression, such reciprocating compressors may utilize a plurality of cylinder-piston combinations in a single compressor unit, may be either single-or double-acting (wherein the piston-cylinder combination discharges higher pressure gas as the piston is stroked in each direction), and may include a two or more stage flow arrangement, wherein the gas is partially compressed in a first stage collection of cylinder-piston combinations and then is elevated to a higher pressure in a second set.
A continuing problem for process designers has been the matching of the particular needs of a process or application with the various models of gas compressors and prime movers available in the marketplace. As will be appreciated by those skilled in the art, such selection must be made as accurately as possible in order to avoid paying for additional, unwanted compressor capacity, or even worse, specifying a compressor-prime mover combination of insufficient capacity for the particular application.
In making such a selection, the designer will typically know the flow rate and pressure of the incoming gas, as well as the desired output pressure. With such information, the required power of the prime mover may be estimated with a fair degree of accuracy, taking into account the mechanical efficiency of the drive coupling, auxiliaries such as cooling, etc. It is typical for most compressor manufacturers to standardize their equipment among a limited number of frames, each frame being suitable for a range of input power. As will again be appreciated by those skilled in the art, purchase of an oversized, heavy compressor frame results in an unnecessarily high capital cost, while use of a compressor frame undersized for the given compressor output will result in an increased number of equipment failures and increased maintenance time and costs.
For typical reciprocating compressors, the choice of a particular compressor frame means that the stroke of the individual pistons is also determined. Thus, in a typical reciprocating compressor application the pressure differential and mass flow of the flowing gas determines the size of the prime mover, the size of the prime mover determines the particular compressor frame, and the choice of compressor frame determines the stroke length of the compressor piston. As most prime movers operate most efficiently at a particular speed, the only remaining variable to the equipment designer is the compressor piston diameter. Compressor manufacturers thus offer a number of different diameter cylinder-piston combinations for use on a particular frame, but the number of such options is finite and often results in a compromise match for a particular desired volumetric gas flow rate.
As will be appreciated by those skilled in the art, a later change in the gas flow rate, pressure differential, or other gas properties may result in a significant mismatch between the application and the existing compressor-prime mover. As discussed hereinabove, it may be possible to adapt the existing unit by swapping cylinders and pistons onto the existing compressor frame, but such changes require new components and such decisions are typically made against the background choices of either purchasing an entirely new compressor-motor unit or providing a new cylinder-piston combination to the existing unit which will still be mismatched to the new application. In such situations a very large mismatch will typically be tolerated before the decision is made to purchase an entirely new unit.
One particular application in which these problems are common is the use of gas compressors in the production of natural gas. Natural gas is typically produced from a number of widespread, remote fields, each field being able to produce only a relatively small amount of gas. Such situations require a number of small compressors, sized to the particular application, and distributed among the individual fields. Not only must each of these compressor-prime mover units operate dependably and cheaply over a long period of time, but it is frequently necessary to alter the flow rate or pressure output of these units as the individual gas fields age or as changing economic conditions result in a different optimum production rate.
What is required is a compressor-prime mover unit which is rugged, dependable, and easily adaptable to changed operating conditions.